Method for attenuating interference in a magnetically shielded room

ABSTRACT

The invention relates to a method for attenuating magnetic interference in a test room ( 1, 2, 8, 9 ) equipped with a shield of ferromagnetic material, in which method the interfering magnetic field is measured and a compensating field is generated on the basis of the measurement result. According to the invention, the interfering field to be measured is resolved in three magnetic field components oriented in different directions, each one of said magnetic field components (x, y, z) is measured with the help of coils ( 3, 4, 5, 6 ) wound about the wall elements ( 1, 2, 8, 9 ) made from said ferromagnetic material in a manner wherein a flux component passing through at least one of the wall element ( 1, 2, 8, 9 ) made from said ferromagnetic material that is oriented parallel to one of said magnetic field components (x, y, z) is measured with the help of coils wound about said wall element.

[0001] The invention relates to a method according to the preamble of claim 1 for attenuating magnetic interference in a magnetically shielded room.

[0002] For high-sensitivity biomagnetic measurements, the test room must be shielded against external interference. As a starting point, it must be understood that the resolution of a modem SQUID magnetometer is in the order of 1 fT/Hz, while typical noise level of a laboratory room is 10₆ fT at 1 Hz and almost 10₉ fT at 0.01 Hz. Although gradiometers are conventionally used in measurements, it is necessary to attenuate the level of external interference fields. Generally, attenuation is accomplished with the help of two or a greater number of shields made from mu-metal. Also active means have been employed to improve the shield. Herein, the external magnetic field is measured outside the test room with the help of a flux-gate magnetometer, for instance, and then coils placed outside the test room have been used to improve the attenuation efficiency of the test room. Also a method has been described, wherein a Helmholtz coil placed outside the test room is used as both a magnetometer and a means for compensating for the detected field.

[0003] It is an object of the present invention to overcome the drawbacks of the above-described techniques and to provide an entirely novel type of method for attenuating magnetic interference imposed on a magnetically shielded room.

[0004] The goal of the invention is achieved by way of arranging about the wall elements of a test room made from a ferromagnetic material a set of measurement coils by means of which the field causing interference can be determined. After the field of interference is known by its components, a compensating field of opposed magnitude can be established by means of compensating coils that adapted outside the test room.

[0005] More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

[0006] The invention offers significant benefits.

[0007] The coil arrangement according to the invention makes it possible to measure the field of interference to a very high accuracy by virtue of making the shielded test room itself to act as the magnetometer. Then, the actual compensation may be carried out with the help of coils placed exterior to the test room.

[0008] A preferred embodiment of the invention also allows the field pattern to be resolved.

[0009] When necessary, even strong interference fields can be attenuated by feedback signals applied to multiturn compensating coils.

[0010] In the following, the invention is described in more detail with reference to an exemplifying embodiment elucidated in the appended drawing in which

[0011]FIG. 1 shows a perspective view of an arrangement for attenuating an interference field.

[0012] Referring to the drawing, a case is assumed wherein the shielded room is comprised of two enclosingly superposed sheet walls made from mu-metal. The construction of the outer wall is illustrated in FIG. 1. For greater clarity of the illustration, two walls of the room, namely the front and the rear wall thereof, are omitted. The external source of interference imposes a magnetic field on the outer wall formed by the bottom and ceiling wall elements 1, 2 and the side wall elements 8 and 9, as well as the front and rear walls not shown in the drawing. If a multiturn coil is wound about the planar wall element 1 or 2, the magnetic field flux circulating in the wall induces a voltage in the coil. Through a series connections of the coils measuring the same vector component of the magnetic field imposed on the parallel-oriented wall elements 1 and 2, a measurement signal is obtained that is proportional to the magnitude of the selected vector component of the imposed field. Accordingly, a single wall 1 can be provided with four coils 5, 6 and 10, 11, respectively at each end of the wall, that measure the magnetic field in two different directions. In the embodiment shown in the drawing, coils 5 and 6 are arranged to measure the field strength in the x-direction, while coils 10 and 11 measure the field strength in the y-direction. Thus, e.g,, the x-direction field component can be measured using coils mounted on four wall elements in total, that is, using coils 3, 4, 5 and 6 and, additionally, using the other set of four coils of the not shown walls that are located substantially in the plane of the drawing. By way of using the same technique for measuring all the three vector components (directed along axes x, y and z) of the magnetic field, it is possible to obtain information on the nature of the external field “entering” the test room. Conversely, by applying a feedback voltage in opposite phase to the voltage induced in the coils that are located outside the test room, whereby coil 7, for instance, can act as the compensation coil of the x-direction field component, a significant reduction can be obtained in the magnitude of the external field reaching the interior of the test room. Complementary to coil 7, the system also has compensation coils for the y- and z-direction field components. In the design of a compensator, it is must obviously be recognized that the induced voltage is proportional to the time derivative of the varying magnetic field.

[0013] While it is possible to implement the concept of the invention having basically only one multiturn coil wound about one wall element per each vector direction x, y and z of the magnetic field. However, using two coils 5 and 6 wound about one wall element 2 as shown in the drawing, it is possible either to obtain a stronger signal from a series connection of the coils or, alternatively, to assess the gradient of the magnetic field between the ends of the wall 2 by way of measuring the voltages induced in the coils 5 and 6 independently. The same strategy may also be applied between the opposite walls 1 and 2 by way of using their coils either for getting a stronger overall measurement signal or, alternatively, for determination of the field distribution between top and bottom or front and back wall of the “tubular” space element. Typically, the measurement signal is maximized by arranging the sensing of the interference field to take place with the help of a tube formed by four walls and having its axis aligned parallel to the field being measured, whereby each wall element has one or more coils wound about the wall element and all the coils being connected in series with each other so that voltages induced in the coils augment each other thus giving a measurement signal proportional to the overall flux of the interference field passing through the wall. The same arrangement can be applied to all the three field components which means that in the case illustrated in the drawing, the x-component of the magnetic field is measured with the help of the tubular space element perpendicular to the magnetic field, the y-component with the help of the longitudinal tubular space element (drawn in a disposition perpendicular to the plane of the diagram) and the z-component with the help of the vertically aligned tubular space element. To eliminate measurement errors, it is even possible to the measure the flux at the ends of the “tubular” space elements with the help of separate coils.

[0014] Man-made magnetic field interference increases toward lower frequencies at a rate of about 20 dB/decade. At very low frequencies or in a low-interference environment, the external interference is caused by geomagnetic fields. These are characterized in that the interference level increases toward low end of the frequency spectrum at a rate of 30 dB/decade. In addition to these, the 50 Hz mains frequency and its harmonics act as a significant source of interference in biomagnetic measurements. It must be borne in mind that the voltage signal in inductively coupled phenomena is proportional to the frequency of the field. At high frequencies, the noise level is generally determined by the thermal noise of the loss components of an antenna, but at low frequencies the noise signal arises from the so-called 1/f noise of the amplifier. More specifically, the signal-to-noise ratio of an antenna integrated with the magnetic shield of a test room falls at a rate of 20 dB/decade over the frequency range of about 0.1 Hz to 1 kHz, while toward the very low frequencies (of about 0.001 Hz to 0.1 Hz) the rate is about 30 dB/decade. This means that the voltage resolution of a multiturn coil integrated in the wall of the test room exceeds over a wide frequency range (0.001 Hz to 1 kHz) the level of voltage induced by an external interference field on a wall. Hence, an active shielding arrangement based on induction coils mounted on the walls of a test room can reduce in a crucial manner the magnetic field penetrating into the test room.

[0015] The implementation of the present method requires for the elimination of the different components of the interfering magnetic field the use of coils mounted outside the test room in same manner as coil 7 in FIG. 1. Typically, the system uses a 50 Hz oscillator which is synchronized to the mains frequency and wherefrom 100 Hz and 150 Hz harmonic signals are generated, whereby the interference level of the test room is reduced through applying all these signals in proper phase to the coils. Antennas mounted on the test room walls can be utilized for measuring the interference signal level at 50 Hz, 100 Hz, 150 Hz and higher harmonics and then controlling the amplitude and phase of the oscillator signal and its harmonics so as to minimize the interfering field.

[0016] If the test room is comprised from several shields enclosingly superposed one another, it is also possible to provide the inner walls of the intermediate enclosures with coils for measuring the magnetic field. In many cases it is not necessary to install new coils inasmuch the room walls anyhow are provided with coils used for demagnetization. However, the inner shields are ill suited for active shielding, because the induced voltage is of the same order of magnitude as the thermal noise voltage of the antenna.

[0017] A practicable feedback arrangement is difficult to implement. This is because the test room in practice tends to be nonsymmetrical and a transfer function between the current applied to the compensation coils and the voltage induced in the coils of the enclosure exhibits a slow response and a complicated nature. For optimized control, the magnetic field induced in the innermost space of the test room by the externally acting magnetic field should be measured using a SQUID magnetometer and the thus obtained information should then be used for optimizing the operation of the compensating controller.

[0018] The arrangement according to the invention may also be used in such shielded rooms that are provided with only a single wall of ferromagnetic material. Herein, the term ferromagnetic material refers to any material of a high relative permeability. Such materials among others are iron and mu-metal. Typically these materials offer a relative permeability of about 100 to 30,000.

[0019] According to an embodiment of the invention, the magnetic field is measured by inductive means from the exterior or interior shielding walls of the test room and this information is used for controlling, typically via a feedback arrangement, coils located outside the room. However, the response of the room is slow, which means that when the current through the coils is changed, it takes several milliseconds before the thus induced magnetic field has penetrated through the walls into the interior of the test room. In the transfer function of system, this retarded response is not simply that caused by one or two poles of the function, but rather is similar to that caused by a delay circuit. In terms of control technology, this means that the interference field imposed on the test room walls cannot be compensated for by feedback means even if the control signal amplitude could basically be made sufficiently high for the compensation. The problem can be solved by complementing the system with a feedforward control. Hence, in a preferred embodiment of the invention, the magnitude of the residual field (substantially at the “higher” frequencies of about 1 Hz to 100 Hz) is measured to detect those components that have not been able to be eliminated by the feedback control applied in the above-described manner to the different coils (x, y and z) of the room and, based on the measurement, a predictive estimate is computed on the field to be in the interior of the room after a short period of time. More specifically, the predictive estimate of the x-direction component of the magnetic field at a point x, y, z of the test room is computed from equation Bx(x,y,z,t)=a(xyz,0)Bx(t)+a(xyz,dt)Bx(t-dt)+a(xyz,2dt)Bx(t-2dt) . . . b(xyz,dt)By(t-dt) . . . c(xyz,dt1)Bz(t-dt) . . . In order to solve the coefficients a(xyz,dt), a(xyz,2dt), a(xyz,3dt) . . . , b(xyz,dt), . . . , the magnetic field at the walls must first be measured and the correlation function of the measurement results must be computed at different points in the interior of the test room. Obviously, this model of the field behavior need to be computed only for those points of the test room where the sensors of a biomagnetic imaging apparatus, typically a brain imaging apparatus, will be situated. In practice it can be assumed that the correlation functions must be computed for all directions of the field at the imaging center of the brain imaging apparatus, however, so that the model can predict values for all the components of the magnetic field and its gradients. Another technique is to estimate the magnetic fields at six points (or in practice, at seven points) for each one of the three axes of the room space, whereby the number of variables to be processed for the predictive estimate is 18 (or 21, respectively). If the system is assembled in its minimum configuration, all the coils of the test room are used and the signals obtained therefrom are combined so that estimates Bx, By and Bz are obtained for the homogeneous field components in the room. These results are then used in a normal manner for minimizing the residual field by feedback control. The measurement signals of the residual field components are sampled, e.g., at 1 ms intervals and the thus obtained matrix of measurement values is used up to, e.g., a period of 1 s for predicting the magnitude of the magnetic field in all directions, e.g., at six (or seven) points of the test room. This information is submitted, e.g., to the computer of the brain imaging apparatus that utilizes the field information when processing the imaging data. Another alternative arrangement is to place coils in the interior of the test room that then compensate for the residual field on the basis of the computed predictive estimate thereof. While the former signal-processing technique probably gives a more accurate end result, the latter has the benefit that it facilitates the use of component constructions delivered by different manufacturers for both the shielded room and the magnetometer.

[0020] In practice the temporal dependence of the residual field can be formulated “mathematically”, thus permitting a substantial reduction in the computation task. The “predictive” estimation of the residual field may also be carried out using recursive methods. Using an efficient algorithm, a conventional PC may accomplish the task effortlessly. 

What is claimed is:
 1. Method for attenuating magnetic interference in a test room (1, 2, 8, 9) equipped with a shield of ferromagnetic material, in which method the interfering magnetic field is measured and a compensating field is generated on the basis of the measurement result, characterized in that the interfering field to be measured is resolved in three magnetic field components oriented in different directions, each one of said magnetic field components (x, y, z) is measured with the help of coils (3, 4, 5, 6) wound about the wall elements (1, 2, 8, 9) made from said ferromagnetic material such that flux component passing through at least one of the wall elements (1, 2, 8, 9) made from said ferromagnetic material that is oriented parallel to one of said magnetic field components (x, y, z) is measured with the help of coils wound about said wall element.
 2. Method according to claim 1, characterized in that each one of the wall elements (1) has two coils (3, 4) at the ends of said wall element (1) for the measurement of one component (x) of the magnetic field.
 3. Method according to claim 2, characterized in that said coils (3, 4) are connected in series in order to maximize the detected signal.
 4. Method according to claim 2, characterized in that said coils (3, 4) are connected to separate circuits in order to detect the field gradient.
 5. Method according to any one of foregoing claims, characterized in that one of the field components (x, y or z) is measured with the help of a tubular structure formed by four wall elements whose axis is oriented parallel to the field component being measured, whereby each one of the walls elements (1, 2) is equipped with at least one measurement coil.
 6. Method according to any one of foregoing claims, characterized in that each one of the compensation coils (7) is located outside the wall elements of the test room.
 7. Method according to any one of foregoing claims, characterized in that an apparatus is used comprising feedback means for adjusting the magnitude and phase shift of the feedback signal to each one of the compensation coils (7) on the basis of the measurement signal obtained from the respective measurement coils (3, 4, 5, 6).
 8. Method according to any one of foregoing claims, characterized in that the interfering field is measured with the help of a SQUID magnetometer.
 9. Method according to any one of foregoing claims, characterized in that the processing of the feedback signal takes into account the dependence of the induced voltage on the time derivative of the imposed magnetic field.
 10. Method according to any one of foregoing claims, characterized in that the residual magnetic field in the test room is measured and, based thereon, a forecast estimate is computed on the field to be in the interior of the test room after a short period of time.
 11. Method according to any one of foregoing claims, characterized in that the correlation function of the measurement results are computed for all directions of the magnetic field at the center point of the measured test room volume, more particularly so that the model used in the computation gives a forecast estimate for both the magnetic field and its gradients in all the computed directions.
 12. Method according to any one of foregoing claims, characterized in that all the coils of the test room are utilized by way of combining the signals obtained therefrom so as to get estimates Bx, By and Bz for homogeneous field components in the test room, whereupon these results are used for minimizing the residual field in the test room by way of a feedback arrangement. 